The main objective of porting a cylinder head is to increase cylinder filling. The more air that is taken into the engine, the more atomized fuel there will be and the more power the engine will produce. Your engine is basically an air pump and just like you it needs to be able to breathe in deeply and breath out quickly. But unlike you the engine also wants to ensure the fuel mix is even, well atomized and getting to the right places in your combustion chamber.
As an approximate guide, for every 10 CFM (cubic foot per minute) increase in intake airflow 5 more horsepowers are possible but more airflow alone does not equal a faster vehicle, you need to create a forcefulness of combustion in order to move large amounts of air quickly and you need to ensure it happens at the right place in the engine’s rpm for the use you intend. For example, old engines, or big engines like Harley’s, there is no point in maxing out at high rpm because these engines don’t go there. Likewise, for most road use in real life circumstance. You want the power where you will use it most.
One of the commonest mistakes, with amateur porting, is a focus on increasing the size of the ports in an attempt to increase quantity of fuel air mix and port velocity. However, it actually causes the flow’s speed to decrease affecting the mixture’s distribution around the cylinder and combustion chamber. This leads to the air and fuel being less thoroughly mixed, especially at lower rpm and will cause the typically reduced midrange power experienced in many “tuned” engines. What one actually wants to achieve is a gain in CFM without losing port velocity.
Any Port in a Storm?
In a perfect world, all intake and exhaust tracts would be straight. This simply is not true in reality, and although even radical modifications can be done to improve standard head designs, mostly it is impossible; especially when you want your engine to fit into a standard or original chassis.
In order to achieve the best results, one then has to try to make the port work as if it is straighter by widening ports at specific points, depending on other factors but in many cases leaving them standard elsewhere, creating a flow that is consistent across the port.
Often ports actually need humps added or dips filled in, especially in older heads. The valve seat area needs to be blended to suit to increase the radii of their section as the mixture enters the chamber. Grinding isn’t the only way to do this. Filling with weld or epoxy liquid metal is often required to achieve the correct objective.
The Importance of the Valve Seat
We often boast that we are a world leading specialist in cylinder head work and this is true. We pioneered the use of the Serdi 100 seat cutting equipment for motorcycles. The experience of doing so, especially working on 1000s of air-cooled engines of all vintages, many of which have irregular castings, has benefited us hugely.
What we have discovered is that careful positioning and working around the valve seat benefits your engine the most. This could mean narrowing it or raising it toward the edge of the valve, and carefully designing and blending the seat’s angles to suit, in three or even five angles. This requires special tools and specialist skills. The idea is not to just spray as much fuel mixture as possible around the chamber, but also to put it in the maximum in right place, where it will burn the best. A three or five angle seat will pull the flow outwards increasing the angle of entry.
As much as 90% of effective porting takes place within an inch or so of the valve seat. How can you port if you cannot control or work on the valve seats? The fact is, many porters cannot.
Polishing is not only not required, often it decreases the effectives of your engine. It was once fashionable, but was in fact for cosmetic purposes designed to impress the eye of the individual paying for the job or look good at trade shows. It does not work to the benefit of the engine. A smooth slippery surface will not assist in the important atomizing mix of the fuel/air mixture. A very rough surface reduces air drag caused by something called the “boundary layer effect”
Polished surfaces accumulate air pressure next to them creating an area of stagnant air which obstructs airflow. A roughened surface eliminates this increasing airflow and helping ensure the mixture enters the combustion chamber thoroughly mixed, which improves combustion and increasing power. Slight roughness keeps fuel droplets suspended in the airstream, so expect to see that when you get your head back.
Lastly, we need to look at how your flow squirls’ (vortex) into your chamber and where your spark or sparks are. This very much differs from head to head, so each head has to be approached individually.
Is your tuner experienced?
Most tuners naturally work and focus on one particular brand for the sake of not your head but their “business”, e.g. Harley-Davidson, or modern performance engines which are increasing identical, very well designed, and hard to improve on. However, at the Cylinder Head Shop, we have the widest possible experience, literally from a century’s worth of engines.
From motorcycles to motor cars, marine to aeronautic, vintage to unreleased prototypes, even generators from nuclear power stations and £500,000 castings … we have worked on, and learned from, them all.
We doubt there is anyone with a wider range of experience.